Spinal disc herniation, a common ailment, and degenerative disc disease often induce pain, as well as neurologically and physiologically debilitating consequences, for which relief becomes paramount. If conservative treatments fail, the more drastic measures of discectomies and spinal fusion may be indicated. The latter treatment, while providing short term relief, limits spinal mobility and often leads to excessive forces on facet joints adjacent to the fusion and may create further problems over time. Drastic treatments are usually unable to restore normal disc function. The loss of disc function has led to a number of disc prostheses that attempt to provide natural motion.
The literature documents that the Instantaneous Axis of Rotation (IAR) during sagittal rotation of the superior vertebra with respect to the inferior vertebra of a Functional Spinal Unit (FSU) in the cervical spine moves significant distances during flexion and extension of the spine (Mameren, H. van, Sanches, H., Beursgens, J., Drukker, J., “Cervical Spine Motion in the Sagittal Plane II: Position of Segmental Averaged Instantaneous Centers of Rotation—A Cineradiographic Study”, Spine 1992, Vol. 17, No. 5, pp. 467-474). This motion can vary widely between functional spinal units on an individual spine and between individuals and can depend on a variety of factors, including age, time-of-day, and the general health and condition of the intervertebral discs, the facet joints and other components of the FSU and spine. A moving IAR means that the superior vertebra is both rotating and translating, while moving with respect to the inferior vertebra of an FSU.
Researchers have attempted to design a successful intervertebral disc replacement for years. Salib et al., U.S. Pat. No. 5,258,031; Marnay, U.S. Pat. No. 5,314,477; Boyd et al., U.S. Pat. No. 5,425,773; Yuan et al., U.S. Pat. No. 5,676,701; and Larsen et al., U.S. Pat. No. 5,782,832 all use ball-and-socket arrangements fixed to the superior and inferior plates, which are rigidly attached to the vertebrae of an FSU. However, these designs tend to limit motion to rotation only about the socket when the ball-and-socket components of the two plates are in contact. As the literature points out (Bogduk N. and Mercer S., “Biomechanics of the cervical spine. I: Normal kinematics”, Clinical Biomechanics, Elsevier, 15(2000) 633-648; and Mameren, H. van, Sanches, H., Beursgens, J., Drukker, J., “Cervical Spine Motion in the Sagittal Plane II: Position of Segmental Averaged Instantaneous Centers of Rotation-A Cineradiographic Study”, Spine 1992, Vol. 17, No. 5, pp. 467-474), this restricted rotational motion does not correspond to the natural motion of the vertebrae, either for sagittal plane motion or for combined sagittal, lateral and axial motion. Further, when the ball-and-socket arrangements on the two plates, as described in the above-cited patents, are not in contact, the devices are unable to provide stability to the intervertebral interface, which can allow unnatural motions and lead to disc related spondylolisthesis, FSU instability, and excessive facet loading.
Advances in disc arthroplasty design have resulted from intense research and patent activity in this field. Duggal in U.S. Pat. No. 7,927,374B2, for example, discloses a design comprising a nucleus between two endplates possessing articulating surfaces with two endplates. Other researchers and designers have disclosed similar structures fitting such a general description, such as for example: Bullivant, U.S. Pat. No. 5,507,816; Erickson, U.S. Pat. No. 6,368,350; Gauchet, U.S. Pat. No. 6,527,804; and Cohen et al. U.S. Pat. No. 7,282,063. However, the nuclei in these examples do not couple flexure joint action with mechanical articulation.
Motion coupling between flexion-extension, axial rotation, and lateral bending and other functional spinal units involved in the overall spinal motion, increases the complexity and difficulty in developing a prosthetic disc replacement that realizes natural spinal motion. Producing such complex disc motions with strictly articulating joints is at best problematic. Alternatives that utilize only a flexible core or nucleus between two plates to solve the problem of disc motion requirements is also not a new idea. Stubstad et al. U.S. Pat. No. 3,867,728 proposes an elastic polymer between plates. Other patents provide similar instruction, for example, see Fuhrmann et al., U.S. Pat. No. 5,002,576 and Baumgartner U.S. Pat. No. 5,370,697. New designs, such as the Spinal Kinetics M6 device based on Reo et al., and U.S. Pat. No. 7,731,753, have been developed.
Additional motion complexity required of a spinal disc derives from motion constraints dictated by facet joints. Complex facet joint surfaces in an FSU can significantly influence and constrain sagittal, lateral and axial motions. The orientation of these facet surfaces varies with FSU location in the spine and allows for wide variations in motion parameters and constraints. The complex motion of a superior vertebra with respect to the associated inferior vertebra of an intact spinal joint segment, certainly in the cervical spine, cannot be realized by a simple rotation or simple translation, or even a combination of rotation and translation along a fixed axis, and still maintain the integrity and stability of the spinal segment, in particular the facet joints.
Natural spinal motions, therefore, can place severe requirements on the design of a prosthetic disc, such that a minimum of six independent degrees of freedom is usually required to achieve full naturalized mobility. The number of independent mechanical degrees of freedom of a disc modeled on rigid body assumptions, however, can be undesirably reduced by one or more degrees of freedom with the use of one or more joint stops or other singularities of configuration, thus limiting further motion along or within the range of motion of the reduced degrees of freedom. Using only mechanically articulating joints, therefore, can be problematic in resolving complex disc motion. Adding flexure motion capability between relatively rigid mechanical joints within a disc prosthesis can enhance the prosthesis responsiveness when in configurations that restrict certain motions of mechanical articulating joints. On the other hand, mechanical articulating joints can provide structurally stronger and more robust motion generation with less stress. A combination of articulating joints and flexure motion structures within this invention disclosure, therefore, takes advantage of each type of joint. This combination of mechanical and flexural joints represents a hybrid disc prosthetic system, hybrid because it employs both rigid-body articulating joints and flexure joints
A natural spinal joint responds to spinal muscle controls primarily through flexure of the disc and mechanical articulation of synovial facet joints. Bone structures, tendons and muscles of a spinal joint also influence spinal joint motion. These complex interactions have yet to be modeled to any degree of accuracy, although progress is being made.
Some recent patents and published applications relating to spinal disc prostheses incorporating flexible elements between endplates in an attempt to emulate natural disc behavior without articulating joints are U.S. Pat. Nos. 8,377,138B2; 7,857,852B2; and U.S. Published Patent Application No. 20090270988A1. While these solutions provide more variability in motion, they are limited in the range of motions provided in any given direction and they do not have the robustness engendered by the use of mechanical elements.
There is a need in the art for a spinal disc prosthetic device that can provide all desired degrees of freedom and natural ranges of motion. Specifically, a device is needed that combines articulating mechanical joints and material flexure in a unique way to accommodate complex spine joint motion. Such a device will solve certain natural motion and shock absorbing characteristics that are problematic for spinal disc prostheses and offer a scalable mechanism for disc replacement without loss of general motion capabilities.
Specific embodiments of such a device might utilize a flexure material that articulates with one or more plates as an integrated whole, and could even possess gradients in mechanical moduli and material structure. In a further ideal, the flexure material would be moveably locked between the plates so as to resist expulsion and separation of the unit. Specifically, the geometry and material properties of the flexure material can couple with the mechanical articulating joints in a fundamental manner that can allow complementary rotations and translations and can, preferably, resist such motions by generating reaction forces and moments-of-force to torsions, compressions, extensions, and bending, as a consequence of its material properties.
Thus, there is a need for a hybrid joint system, being one that employs rigid-body-modeled, articulating, mechanical joints with a non-rigid body flexure nucleus between mechanical joints, which can perform complex, coupled motions required of a natural disc.
The invention herein discloses devices that can perform such complex motions without knowing the precise motion trajectories produced by a natural disc. Specifically, articulating mechanical joints of the invention can provide four independent degrees of freedom. Flexure of the nucleus between the two mechanical joints, allows a continuum of motions within the rated modulus of the material. In particular, flexure in the invention accommodates compression-extension and lateral bending motions along the axial axis of the FSU independent of the articulation joint degrees of freedom.